With LTE Rel 8, a transmission band of 20 MHz was defined. In LTE-Advanced carrier aggregation will be used to support system bandwidths up to 100 MHz. This involves splitting the system bandwidth into five 20 MHz sub-bands, each centred on a respective component carrier. In order to be backwards compatible with LTE Rel 8 User Equipment (UEs), at least one of those sub-bands has to be LTE Rel 8 compliant.
To support carrier aggregation a new Physical Downlink Control Channel (PDCCH) structure is needed to address the aggregated system bandwidth of up to 100 MHz. Two different approaches, separate PDCCH for each carrier and common PDCCH for multiple carriers, are under consideration in RAN 1. One of these two methods will be agreed to be adopted for the LTE-Advanced System. Irrespective of this decision, at any given point in time the UE may not transmit or receive data on all the component carriers. The base station (referred to as the eNB in LTE documentation) should have the flexibility to schedule a UE on any of the component carriers it wishes and should be able to move the UE on to different component carriers as required. The UE on the other hand should know in advance which subset of the component carriers it should monitor and turn off its RF transceiver circuitry for the others. This would result in considerable power savings in the UE.